1. Field of the Invention
Ammonium phosphates produced by the reaction of ammonia with phosphoric acid became the leading phosphatic fertilizers produced in the United States in the late 1960s and since that time their manufacture and use have continued to increase. Both diammonium and monoammonium phosphates are commonly produced. Initially both of these fertilizers were produced using a slurry type process such as that taught in U.S. Pat. No. 3,153,574, Achorn et al, assigned to the assignee of the present invention, which was designed principally for production of diammonium phosphates. Other processes were developed that offered distinct advantages when producing monoammonium phosphates. These processes involved a new technique of granulation called melt granulation in which an inline type reactor, such as a pipe reactor or pipe-cross reactor, was used to produce an essentially anhydrous ammonium phosphate melt. This allows much better conservation of heats of reaction and thus reduces or totally eliminates any drying requirement or step during subsequent treatment of fertilizer material exiting the granulator. Cost savings are realized because fuel requirement for heating is decreased and capital investment requirements are also decreased because complicated slurry production and handling equipment, such as a preneutralizer, and drying equipment are no longer required. Thus, in summary, operating costs are decreased because not only fixed but also variable expenses are decreased.
2. Description of the Prior Art
Several processes have been developed using the melt granulation technique including those taught in U.S. Pat. No. 3,825,414, Lee et al, assigned to the assignee of the present invention, where a pug mill granulator is used; in U.S. Pat. No. 3,985,538, Hicks et al, also assigned to the assignee of the present invention, where a drum granulator is used, but a preneutralizer is also used in the reaction system. There are certain inherent disadvantages to these two processes, however. The former process used not only a complicated reaction system that included a spray reactor and vapor disengager in addition to the pipe reactor, but also used a pug mill granulator which is typically more expensive to operate and maintain than a drum granulator and is much less commonly used in the fertilizer industry. The latter process used a drum granulator but also had a much more complicated reaction system whereby a preneutralizer tank is also included with the pipe reactor.
Later processes were developed that were more suited to production of ammonium phosphate fertilizers in drum granulators. In U.S. Pat. No. 3,954,942, Achorn et al, assigned to the assignee of the present invention, taught the use of a pipe-cross reactor as the reaction system. With this type of inline reactor, sulfuric acid can also be fed and co-neutralized along with the phosphoric acid inside the reactor so that a fertilizer can be produced that contains additional sulfate, if needed. This process was improved, as taught in U.S. Pat. No. 4,134,750, Norton et al, assigned to the assignee of the present invention. Norton et al modified the earlier process of Achorn, supra, so that higher temperatures could be maintained in the pipecross reactor and ammonium phosphate fertilizers containing polyphosphate were produced. Although both the Achorn and Norton processes, to wit, '942 and '750, supra, obtain a granular product without external heat; both utilize sulfuric acid fed to the pipe-cross reactor to furnish the required and necessary additional chemical heat to the process.
The Lee process, to wit, U.S. Pat. No. 3,825,414, supra, produced a product in which a 20-percent polyphosphate content was the minimum acceptable for granular material which is intended to be subsequently used as an intermediate for production of suitable suspension fertilizers. In this process, a pug mill granulator was used. This type of granulator has not been well accepted by the granular fertilizer industry. The pug mill is a complicated piece of granulation equipment and because of this it has a higher investment cost than other more common granulators, such as a drum granulator, and is more difficult to maintain in good operating condition, thus increasing maintenance costs. Since more energy, especially electrical energy, is required to operate the pug mill, operating costs are further increased. Because a pug mill cannot be easily or readily hooded, it is more difficult to scrub the fumes from a pug mill and pollution control is more expensive. Investment costs and operating costs are higher because of the use of the pug mill granulator and because of the more complicated reaction system comprising a spray reactor, vapor disengager, and pipe reactor. For a better understanding of the theory, construction, and operation of the spray reactor and vapor disengager, see FIGS. 5 and 6 of U.S. Pat. No. 3,733,191, Meline et al, assigned to the assignee of the present invention, in which the spray reactor is referred to as the first-stage reactor and the vapor disengager as a horizontal tube and rotor-type disengager. The spray reactor must be operated at high temperatures and low pH so that the material of construction is a material such as Hastelloy G that is not normally used in the industry, especially for large pieces of equipment, because of its high costs. The vapor disengager is a rather complicated piece of equipment to operate and is considered by some to be above the normal state of the art practiced by the industry. Parts of this vapor disengager would be expected to require an expensive material of construction. In addition, maintenance costs are high because of the rotary nature of the disengager and close tolerances maintained so that highly skilled maintenance personnel are required.